In recent computer systems, as well as high speed and high performance processing devices, there has also been a demand for improvements in the transfer rate of a system bus which is the transmission path for the results of processing. Also, accompanying the diversification of systems, there has been an increasing necessity to connect electronic circuits having various functions mainly to the system bus. The roles played by computer systems have become much more serious, and in order to maintain expansion devices including the aforementioned electronic circuits, there is a tendency to not permit system stoppages, and a need has arisen to connect or disconnect these expansion devices using active lines. On the other hand, in shared bus system type configurations, there is also a need for a way of settling the bus signal waveform quickly, for each expansion device, in order to achieve high speed operation.
Conventionally, as disclosed in Japanese Patent Laid-open Publication No. Hei 5-12777 (which will be referred to as publication 1), there has been known a device in which power is supplied beforehand using a cord, etc., and disable control (open control) of bus drivers is carried out. As disclosed in Patent laid-open publication No. Hei 4-171520 (publication 2) a power supply and an open/close control line of a driver are connected to bus circuits by a long pin, while other bus signal lines are connected to the bus circuits by a short pin. At the time of insertion of a device, corruption of signals on the bus caused by the insertion is prevented by disabling the open/close signal line of the drivers in advance. However, in publication 1, the procedure of connecting the cord beforehand at the time of insertion or removal is complicated, while in publication 2 an open/close control line must be disabled beforehand. In either case, since it can not be guaranteed that the drivers will be disabled when insertion or removal is carried out carelessly, there is a possibility of problems arising such as the system crashing, for instance.